What laboratory tests are indicated for a patient presenting with edema?

Laboratory Evaluation for Edema

Order a comprehensive initial laboratory panel including complete blood count, urinalysis, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, fasting blood glucose, lipid profile, liver function tests, and thyroid-stimulating hormone for all patients presenting with edema. 1, 2

Core Initial Laboratory Tests

The following tests should be obtained in all patients with edema, regardless of suspected etiology:

• Complete blood count (CBC) to assess for anemia that may contribute to high-output heart failure 2
• Urinalysis with urine protein-to-creatinine ratio to evaluate for proteinuria suggesting nephrotic syndrome or renal disease 1, 3
• Serum electrolytes including calcium and magnesium to identify electrolyte imbalances accompanying edema 1, 2
• Blood urea nitrogen (BUN) and serum creatinine to assess renal function and identify kidney disease as a cause 1, 3
• Fasting blood glucose (or glycohemoglobin) to screen for diabetes-related complications 1
• Liver function tests and serum albumin to evaluate for hepatic synthetic dysfunction and cirrhosis 1, 2, 3
• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism as a reversible cause of edema 1, 2, 3
• Lipid profile as part of comprehensive cardiovascular risk assessment 1

Additional Testing Based on Clinical Context

For Suspected Cardiac Etiology

When heart failure is suspected based on clinical presentation:

• Brain natriuretic peptide (BNP) or NT-proBNP to evaluate for heart failure; elevated levels warrant further cardiac workup 2, 3
• 12-lead electrocardiogram to evaluate for cardiac abnormalities 1, 2
• Chest radiograph (PA and lateral) to assess for pulmonary congestion and cardiomegaly 1, 2
• Two-dimensional echocardiography with Doppler if BNP is elevated or clinical findings suggest heart failure, to assess left ventricular ejection fraction, chamber size, wall thickness, and valve function 1, 2

For Suspected Hepatic Etiology

When liver disease is suspected:

• Serum albumin levels as low levels indicate decreased hepatic synthesis 2
• Prothrombin time/INR as prolongation indicates impaired hepatic synthetic function 2
• Abdominal ultrasound to evaluate for ascites, splenomegaly, and portal hypertension 2

For Acute Unilateral Lower Extremity Edema

This presentation requires immediate evaluation for deep venous thrombosis:

• D-dimer test or compression ultrasonography to rule out venous thromboembolism 3

For Chronic Bilateral Lower Extremity Edema

• Duplex ultrasonography with reflux to diagnose chronic venous insufficiency 3

Critical Pitfalls to Avoid

• Do not fail to review medications including calcium channel blockers, NSAIDs, thiazolidinediones, and hormonal therapies, as these commonly cause edema 2
• Do not neglect thyroid function testing as hypothyroidism is a reversible cause of edema 2
• Do not order basic metabolic panel alone without the comprehensive evaluation outlined above, as this misses critical diagnostic information 1, 2
• Monitor electrolytes closely if diuretic therapy is initiated to avoid dangerous imbalances 2

Algorithmic Approach

1. First-line testing for all patients: CBC, urinalysis with protein-to-creatinine ratio, comprehensive metabolic panel (electrolytes, BUN, creatinine, glucose), liver function tests with albumin, TSH, lipid profile 1, 2, 3

2. If bilateral edema with dyspnea or orthopnea: Add BNP/NT-proBNP, ECG, chest X-ray 2, 3

3. If BNP elevated or cardiac symptoms present: Proceed to echocardiography 1, 2

4. If acute unilateral leg edema: Immediately obtain D-dimer or compression ultrasound 3

5. If chronic bilateral leg edema: Consider duplex ultrasound with reflux for venous insufficiency 3

6. If low albumin with ascites: Add prothrombin time and abdominal ultrasound 2